Issues exist in the manufacturing process for an image sensor. For instance, the thickness of a BEOL (back end of line) layer has increased due to a complicated logic circuit for realizing small-sized pixels having high resolution. In addition, as the pixels have been scaled down, there is a limitation in reducing the vertical distance between a microlens and a photodiode.
The minimum size of a light spot focused on a photodiode is limited to the size of the Airy disc, and the size of the Airy disc is inversely proportional to the diameter D of the microlens. However, the size of the Airy disc increases as the pixel size is reduced, so the minimum size of the light spot focused on the photodiode approximately reaches the pixel size.
The size of the Airy disc defined by f(1/D) is proportional to the focal length f of the microlens. Thus, the simplest method for solving the above problems is to shorten the focal length f by reducing the vertical distance between the microlens and the photodiode. However, this method may not be easily achieved because the thickness of the BEOL layer of the logic circuit is gradually being increased. Thus, the image sensor having the conventional structure cannot simultaneously solve the above two problems.
In order to solve the above problems, an array etch back scheme has been proposed. According to this scheme, the BEOL layer formed on a pixel area is exclusively subject to the array etch back process. However, when this process scheme is actually applied, non-uniformity of center-to-edge lens height and gap CD may occur due to a pixel edge where high step difference occurs during a lithography process for forming the microlens.
In addition, photoresist residue, which is generated when microlens photoresist is coated, may randomly remain at sidewalls.
Further, ‘best focus’ of the lithography process is set to the pixel area where a lens is formed, so photo keys (e.g., overlay boxes), which are formed on an area above the pixel area by about 1 μm, may be severely defocused so that the photo keys are abnormally formed. Thus, normal in-line measurement through the photo keys may be difficult.